This invention relates to methods and apparatus for testing armatures. More particularly, the invention relates to methods and apparatus for ensuring that armature coils and coil lead connections to armature commutators are formed properly.
It is known to test the coils and coil lead connections of armatures by measuring the resistance of the armature coils and coil leads. If the resistance of a coil or coil lead is determined to be too high, the armature can be rejected. One such apparatus for testing armatures is shown in U.S. Pat. No. 4,651,086, which describes a machine that measures the resistance of coils and coil lead connections using an arrangement in which the voltage at two separate leads is equalized using a feedback circuit. Because the voltage at these two leads is the same, current is prevented from flowing through a number of the coils, which allows the resistance of an individual coil lead connection to be measured. However, it is not possible with the arrangement described in the aforementioned U.S. Pat. No. 4,651,086, to measure more than one coil lead connection resistance and one coil resistance measurement to be made at a time. To measure N coil lead connection resistances and N coil resistances requires N measurement steps. Because each step of measuring a set of resistances (i.e., a coil lead connection resistance and a coil resistance) requires a finite minimum amount of time, it would be desirable to increase the throughput of such armature testers by reducing the number of measurement steps that must be performed without significantly increasing the time required for each measurement step.
An apparatus for testing an armature with fewer measurement steps is shown in U.S. Pat. No. 5,140,276. However, the apparatus described in U.S. Pat. No. 5,140,276 does not equalize the voltage at a pair of lead connections as is done with the arrangement described in the aforementioned U.S. Pat. No. 4,651,086. As a result, there is an undesirable current flow through some of the coils, which reduces the accuracy of the resistance measurements. A further disadvantage of the testing apparatus described in U.S. Pat. No. 5,140,276 is that it does not allow the individual measurement of coil lead connection resistances. Rather, pairs of diametrically opposed lead connection resistances must be measured together. Because of this limitation, the testing apparatus of U.S. Pat. No. 5,140,276 does not determine the status of each individual coil lead connection, which can lead to inaccuracies. For example, because only the average resistance of two lead connections is known, occasionally, one of the two resistances will be unusually low and will inadvertently compensate for the abnormally high resistance of the other lead connection in the pair. As a result, the testing apparatus of U.S. Pat. No. 5,140,276 may fail to detect certain defective lead connections that would have been detected if the lead connection resistances were measured individually.
It would therefore be desirable to provide an improved method and apparatus for armature testing that allows individual coil lead connection resistances to be measured and that allows the coil lead connection resistances and coil resistances of an armature to be measured in fewer steps than has previously been possible.
It is thus an object of the present invention to provide a method and apparatus for testing armatures, whereby individual armature coil lead connection resistances and armature coil resistances are measured in approximately half the number of steps that have previously been required.